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Univ ersity of Louisville Univ ersity of Louisville ThinkIR: The Univ ersity of Louisville's Institutional Repository ThinkIR: The Univ ersity of Louisville's Institutional Repository College of Ar ts & Sciences Senior Honors Theses College of Ar ts & Sciences 5-2015
Swimming beha
vior in temperate forest ants. Swimming beha vior in temperate forest ants. Sar ah Frances Handlon University of Louisville F ollow this and additional works at: https:/ /ir.library.louisville.edu/honors P art of the Animal Sciences Commons, and the Biology Commons
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Swimming Behavior in Temperate Forest Ants
By
Sarah Frances Handlon
Submitted in partial fulfillment for the requirements for Graduation (summa or magna) cum laude and for Graduation with Honors from the Department of Biology
Universiy of Louisville
May, 2015
Swimming behavior in temperate forest ants
Sarah Handlon
Department of Biology, University of Louisville
Undergraduate thesis
Abstract
Most terrestrial arthropods are helpless in water, and falling from a tree into a flooded forest understory should be especially problematic for small, cursorial organisms like ants. Whereas many species of tropical arboreal ants can tread across the water surface (i.e., swim), less is known of this behavior in temperate forest ants. I tested for swimming ability in various ant species collected from tree trunks in Kentucky. Results show that Camponotus pennsylvanicus, and C. nearcticus, are strong swimmers (operationally defined as directed motion at speeds > 3 body lengths per sec.), while Crematogaster ashmeadi, and Monomorium minimum tend to struggle and become trapped at the water surface. Laboratory studies suggest that the ants direct their swimming toward dark objects (i.e., skototaxis), presumably to locate tree trunks or other emergent structures. Collectively, these results suggest that living and foraging well above the ground poses special challenges for cursorial animals.
Introduction
Ant colonies function as a social unit (Gordon, 2010). There is a hierarchy system, and each caste is vital to the survival of the colony as a whole. A caste is defined as a specific level in the colony hierarchy (Gordon, 2010). Taking care of young, maintaining the nest, foraging, and defense are examples of different tasks. Workers, the small females that do not reproduce, are responsible for taking care of the young and also for foraging for supplies to sustain the rest of the colony (Delgado, 2000). Ants colonize on the ground, as well as in the trees. Some species forage both on land and in trees, and navigate easily between the two habitats. Certain ant species like Camponotus, otherwise known as the carpenter ant, place their nests and also forage high in the canopy of trees in the tropical rainforests of Central and South America, with no need to come down from the tree to the forest floor (Baader, 1996). Arboreal ant species spend all of their time in the tree. However, there are circumstances beyond the ant's control. For example, winds moving the branches high in the canopy where these ants are commonly found may cause them to lose their connection with the tree and plummet to the forest floor. Likewise, when birds or mammals forage in the tree, the movement of the branches causes ants to fall to the ground below (Haemig, 1997). In the tropical rain forests of South America, the ants could potentially land in the leaf litter, a great distance from their nest with a low likelihood of returning (Yanoviak et al,
2005). During the wet season however, the forest floor is usually covered in
water. Aquatic insects like the water striders (family Gerridae), are an example of an aquatic surface dwelling species that would have no trouble falling onto a rain covered forest floor (Milne and Milne, 1978). Unlike terrestrial land species, striders are capable of navigating successfully across the water without being trapped. This neustonic behavior is possible because their legs do not sink down into the water, but manage to stay on the surface, enabling them to maneuver, change direction and transport themselves on the forest floor when it is covered. In contrast to water striders, moths, bees, butterflies and various other small, terrestrial creatures are trapped by the water. Water's basic property of surface tension prevents their escape. Moths and bees as well as butterflies find that their wings are trapped; while others simply struggle and lack forward motion or directionality. They are eventually caught by underwater predators (Small et al,
2013) or die by drowning. Some spiders, however, have mastered the ability to
navigate on the water's surface without being entrapped. Certain species of land spiders like those found in the Agelenidae family are able to remain dry and move on the water's surface similar to how they maneuver on land (Stratton et al,
2004). Ants are a terrestrial surprise, in that they too possess the ability to evade
almost certain death that comes with falling into the waters below. Certain ant species in the tropics bypass falling into the leaf litter or onto the water's surface entirely by displaying directed aerial descent, or gliding behavior (Yanoviak et al,
2005). Most species of the Pseudomyrmecinae family and Cephalotes genus in
South America display this gliding behavior. These ants will glide not only when dislodged from the tree, but also to avoid predators as well. Not every ant species possesses the ability to glide; some ant species display behaviors similar to that of the aquatic dwelling water strider. Certain arboreal ant species actually swim when they land on the water surface. South American ant species in the families Dolichoderinae and Formicinae displayed strong swimming behavior on the surface of the water (Yanoviak and Frederick,
2014). Certain species have better success than others, and that is correlated to
ant size. Larger ants have greater success at navigating on the water surface without being affected by surface tension (DuBois & Jander, 1985). Smaller ant species like Monomorium minimum struggle with the tension effects and end up flailing at the water surface (DuBois & Jander, 1985). In contrast, when other ant species, like Camponotus pennsylvanicus, hit the surface of the water, they almost immediately begin moving (swimming) towards a nearby object, such as the base of a nearby tree. This directionality displayed by ants in both North and South America is possibly linked to skototaxis. Skototaxis is defined as the ability to navigate to darker objects that contrast with the overall background (DuBois & Jander,
1985). Several different terrestrial and aquatic species are known to exhibit this
behavior. Beetles that live in the leaf litter use skototaxis to help navigate to tree trunks during the flooding that occurs seasonally in the Amazon (Irmler, 1973). Adults of three different flat bug species also showed positive orientation towards dark objects (Taylor, 1988). Skototaxis behavior is beneficial for terrestrial species, like ants, that do not possess gliding capabilities. When the ant first hits the water, it is a prey item for underwater predators. Swimming to the nearest dark object will potentially provide the ant with an avenue for escape, as long as the ant can swim quickly. This behavior allows them to survive in foreign habitats, and presumably assists them in evading predators while they return to the base of the tree. Neustonic qualities are present in tropical ant species, and the same is true of some species of arboreal ants in North America (DuBois & Jander, 1985). Nothing is known about Kentucky species of ants in terms of swimming ability or directionality. Many genera of ants are found both here and in tropical rain forests (e.g., Camponotus, Crematogaster; Yanoviak, 2006), but few studies have compared their behaviors. Although most tree-dwelling ants in Kentucky are unlikely to land in a body of water filled with predators, the same can not be said for all North American ants. The general goal of my research was to explore swimming behavior in local arboreal ants. Within that goal, I focused on the following questions: 1) which ant species exhibit swimming behavior?, 2) how fast do they swim?, 3) does swimming speed change with body size?, and 4) which swimming species exhibit skototaxis? I predicted that the two species of Camponotus: C. nearticus and C. pennsylvanicus would both exhibit strong swimming behaviors and show skototaxis behavior. These predictions were based on the previous work of Yanoviak & Frederick (2014) and DuBois &
Jander (1985).
Methods
This research was conducted in Louisville, Kentucky. All ants were taken from campus trees at the University of Louisville, which is located about two miles south of the city center. The campus is set in an urban environment, with over 2,000 trees of varying types. The city of Louisville averages 31 degrees C during the summer months-- i.e., when my samples were being collected. Only trees with ant activity were surveyed, and all ants were collected on warm, mostly clear days because ants were not active during rain. My first objective was to determine which ant species exhibit swimming capabilities. These trials were conducted in the field. A small, rectangular pan about 25 mm deep was used as a swimming arena. The water depth in the pan was about 13 millimeters. Worker ants were collected by placing baits (tuna mixed with honey) on tree trunks about 1.5 m above the ground. Foraging ants were captured with an aspirator, or with tweezers depending on the ant's size. An aspirator (Figure 1) uses mouth suction to draw an ant through a tube that ends at a vial. This vial was then stoppered until the ant could be tested. Extreme care was taken to ensure that the ant was not damaged during the capture. This was done to guarantee their swimming ability was not adversely affected. After the ant was captured, it was transferred to a vial coated in Teflon. The Teflon creates a non-stick surface to prevent escape. This coating also enabled me to simulate a natural fall by simply inverting the vial over the white pan and allowing the ant to fall into the water. Once an ant hit the water surface, its swimming abilities were measured and categorized into one of the three groups: strong, weak, or non-swimming. Following Yanoviak and Frederick (2014), strong swimmers were classified asquotesdbs_dbs4.pdfusesText_7
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